present invention relates to a device for supplying radiant energy of a wavelength in the near infrared onto a printing substrate in a planographic printing press, including at least one radiant energy source whose light impinges on the printing substrate on the path of the printing substrate through the printing press at a position downstream of at least one printing nip in a printing unit. Moreover, the present invention relates to a method for supplying radiant energy of a wavelength in the near infrared onto a printing substrate in a planographic printing press.
Depending on the type of printing ink and the underlying particular drying process, printing machines, in particular, planographic printing presses such as lithographic printing presses, rotary printing presses, offset printing presses and the like, which process sheet or web stock, in particular, paper, paperboard, cardboard and the like, are known to have different devices which cause or assist the ink to adhere to the printing substrate by supplying radiant energy to the printing ink present on the substrate.
The so-called “UV inks” are cured by polymerization which is induced by photoinitiation using light in the ultraviolet. On the other hand, solvent-containing printing inks which can be subject to both a physical and a chemical drying process are widespread. The physical drying includes the evaporation of solvents and the diffusion into the printing substrate (absorption), whereas “chemical drying” or “oxidative drying” are understood to mean drying due to the polymerization of the oils, resins, binding agents or the like included in the ink formulations, possibly with the participation of atmospheric oxygen. Generally, the drying processes are dependent on each other since, because of the absorption of the solvents, a separation between solvents and resins occurs within the binder system, as a result of which the resin molecules can get close to one another and, possibly, be polymerized more easily.
For example, European Patent Application No. 0 355 473 A2 describes a method for drying printed products which includes a radiant energy source in the form of a laser. The radiant energy is directed onto the surface of the printing substrates, which are moved by a transport device on a path through the printing press, at a position between individual printing units or downstream of the last printing unit before or in the delivery. In this context, the radiation source can be a laser in the ultraviolet for UV inks or a laser in the infrared for heating solvent-containing printing inks. The radiant energy source is located outside the printing press to prevent unwanted heating of parts of the printing press due to heat losses which cannot be avoided or screened off. In this context, the fact that an additional system component must be separately provided to the user of the printing press is a disadvantage.
Moreover, it is known, for example, from U.S. Pat. No. 6,026,748 that a printing press can be provided with a dryer system featuring infrared lamps which emit short-wave infrared light (near infrared) or medium-wave infrared light. Lamp light sources have a wide-band emission spectrum, offering a multitude of wavelengths. Such dryer systems in the infrared have the disadvantage that a relevant portion of the energy is absorbed in the paper, heating the ink only indirectly. Rapid drying is only possible with a correspondingly high energy input. However, inter alia, there is a risk for the printing substrate to dry unevenly and buckle in the process.
In electrophotographic printing, it is known, for example, from German Patent Application No. 44 35 077 A1, to fix toner on a recording medium using radiant energy in the near infrared which is emitted by diode lasers. By using a narrow-band light source, the toner particles are heated in order for them to melt, form a color layer, and to be anchored to the surface of the recording medium. Since a great number of common paper types have broad absorption minima in this spectral range, it is possible for a predominant part of the energy to be directly absorbed in the toner particles.
However, simple knowledge of the window in the paper's absorption spectrum cannot be immediately made use of in printing with solvent-containing printing inks since, as described above, this technology is based on other chemical or physical drying processes. In the context of the present invention, the term “solvent-containing printing ink” is meant to refer, in particular, to inks whose solvent components can be of aqueous or organic nature and which are based on binder systems that are oxidatively, ionically or free radically polymerizable. The input of energy for drying solvent-containing printing inks is intended to have the effect of assisting or promoting the evaporation of the solvent and/or the effect of absorption into the printing substrate and/or the effect of polymerization, while at the same time avoiding unwanted side effects such as, in particular, excessive heating of the solvent-containing printing ink which can result in decomposition of components or in overheating of the solvent. Unlike in the case of toner fixing, the energy input is not only intended for the melting of particles.